1. Field of the Invention
The invention relates to an apparatus and method for reducing contaminants from industrial processes. More particularly, the invention is directed to a method of sequestering pollutants from flue gases in operational plants.
2. Discussion of the Related Art
There are a number of related art methods and apparatuses for reducing contaminants from industrial processes. For example, some potential processes for minimizing or capturing flue gas CO2 emissions include an integrated gasification combined cycle (IGCC), membrane separations, sorbent technologies involving pressure or temperature swing processes, and the use of solvents such as monoethanolamine. See e.g., Herzog, An Introduction to CO2 Separation and Capture Technologies, Energy Laboratory Working Paper, Massachusetts Institute of Technology, August 1999, pp. 1-8; Reynolds, et al., New Pressure Swing Adsorption Cycles for Carbon Dioxide Sequestration, Adsorption 11, 2005, pp. 531-536; Atimtay, Cleaner energy production with integrated gasification combined cycle systems and use of metal oxide sorbents for H2S cleanup from coal gas, Clean Products and Processes, 2001, 2, pp. 197-208; Kintisch, Power Generation: Making Dirty Coal Plants Cleaner, Science, 2007, 317, pp. 184-186. These processes have limitations for widespread practical use as they require capture, separation, and compression of CO2 from flue gas. In addition, concentrated CO2 must be transported to a site where it can be disposed of safely, thereby adding additional cost and potential reemission of contaminants.
Some other related methods for CO2 disposal include storage of CO2 in deep aquifers; injection into saline, oil, and gas reservoirs; and mineral carbonation. See, e.g., Lackner, A guide to CO2 Sequestration, Science, 2003, 300, pp. 1677-1678. Implementation of the mineral carbonation process is not suitable for practical industrial applications as it requires a pure (99.9%) carbon dioxide stream for the reaction. Therefore, the process is not a scaleable process, is energy intensive, and also has a slow mass transfer rate.
Moreover, some other related art methods are directed at sequestering CO2 via a mineral carbonation process involving reaction of CO2 with silicate minerals (e.g., calcium, magnesium, aluminum, and iron) and industrial alkaline solids and precipitation of CO2 into carbonate minerals. For example, in a natural weathering process silicate minerals are converted to carbonate minerals by absorbing atmospheric CO2. Similarly, alkaline solids also carbonate naturally because these solids contain thermodynamically unstable oxides, hydroxides, and silicate minerals which can capture and convert CO2 into carbonates. However, the natural mineral carbonation process of silicates and industrial solids is slow and difficult to implement on an industrial scale.
Other related art processes are directed towards a re-carbonation process in which mineral carbonation of silicate minerals and industrial solid wastes is performed. See Reddy, et al., Solubility relationships and mineral transformations associated with recarbonation of retorted oil shales, J. Environ. Qual., 1986, 15, pp. 129-133. In this study an aqueous recarbonation process was used in which CO2 was bubbled through oil shale solid waste over a period of about 6 months. The bubbling CO2 dissolved silicate minerals and precipitated calcite in oil shale solid wastes. The study suggested that the carbonation process improved the chemical quality of oil shale solid wastes by reducing the concentration of toxic elements. However, the process is not suitable for practical industrial applications as it has long reaction times, e.g., more than 6 months and requires ⅓ or ¼ ratio of water to improve reaction rate, it requires a pure (99.9%) carbon dioxide stream, it is energy intensive and it is a batch process.
Humidity cell carbonation is another related art process. In this process oil shale is exposed to solid waste at a CO2 pressure of about 0.004 MPa and a moisture content of about 70 to 80 percent for about 4 days to determine the effect of carbonation on the availability and plant uptake of trace elements. See Reddy, et al., Availability and plant uptake of trace elements from recarbonated retorted oil shale, J. Environ. Qual., 1987, 16, pp. 168-171. Studies using cell carbonation suggest that the carbonation process promotes plant growth in oil shale solid wastes by increasing the availability of nutrients and by decreasing the toxicity of trace elements. However, the process is not suitable for industrial applications as it has long reaction times, e.g., 4 days, requires pure (99.9%) carbon dioxide stream, energy intensive, requires water addition and is a batch process.
Another related process tried to improve industrial application by accelerating the mineral carbonation process of solid wastes. See Reddy, et al., Effects of CO2 pressure process on the solubilities of major and trace elements in oil shale wastes, Environ. Sci. Technol., 1991, 25, pp. 1466-1469. In this related art process moist oil shale solids were exposed under a CO2 pressure of about 5 psi for about 1 hour. The moist oil shale solids were about 15 to about 20% by weight in the process. This process is not suitable for practical industrial applications as it requires a pure (99.9%) carbon dioxide steam and is a batch process.
Other related art processes have tried to accelerate mineral carbonation conditions for conventional coal, clean coal technology, and oil shale solid wastes. See e.g., Reddy, et al., Reaction of CO2 with alkaline solid wastes to reduce contaminant mobility, Water Research, 1994, 28, pp. 1377-1382; Reddy, et al., Reaction of CO2 with clean coal technology solid wastes to reduce trace element mobility, Water, Air, Soil Pollut., 1995, 84, pp. 385-398; Reddy, et al., Electric Power Research Institute (EPRI), Palo Alto, Calif., 1995, TR-104840, pp. 1-36. Each of these processes has a number of drawbacks including requiring high pressures, requiring pure (99.9%) carbon dioxide streams, energy intensive and are conducted in batch process.
In addition, some other related art processes have applied accelerated mineral carbonation to different industrial residues. For example, these related art processes used municipal solid waste incinerated fly ash/bottom ash from Netherlands, Sweden, Japan, and South Korea in their carbonation process. See e.g., Meima, et al., Carbonation processes in municipal solid waste incinerator bottom ash and their effect on the leaching of copper and molybdenum, Applied Geochemistry, 2002, 17, pp. 1503-1513; Ecke, et al., A. Carbonation of municipal solid waste incineration fly ash and the impact on metal mobility, J. Environ. Eng., 2003, 129, pp. 435-440; Kim, Evaluation of pre-treatment methods for landfill disposal of residues from municipal solid waste incineration Waste Management and Research, 2003, 21(5), pp. 416-423; Ji-Whan, et al., Characteristic of Carbonation Reaction from Municipal Solid Waste Incinerator Bottom Ash as a Function of Water Content and Their Effect on the Stabilization of Copper and Lead, Materials Science Forum, 2007, 544-545, pp. 533-536. Other related art processes have used municipal solid waste incinerated ash and air pollution control residue from United Kingdom. See Fernandez, et al., A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO2, Journal of Hazardous Materials 2004, 112 (3), pp. 193-205.
Other related art processes have used steel slag from the Netherlands. See Huijgen, et. al., Mineral CO Sequestration by Steel Slag Carbonation, Environ. Sci. Technol., 2005, 39 (24), pp. 9676-9682; Huijgen, et. al., Carbonation of Steel Slag for CO2 sequestration: Leaching of products and reaction mechanisms, Environ. Sci. Technol. 2006, 40, pp. 2790-2796; Another related art process has used paper mill ash from Spain. See Perez-Lopez, Carbonation of alkaline paper mill waste to reduce CO2 greenhouse gas emissions into atmosphere, Applied Geochemistry, 23, 8, 2008, pp. 2292-2300. Still another related art process has used hospital solid waste incinerated ash from Italy. See Baciocchi, et al., CO2 sequestration by direct gas-solid carbonation of APC residues, Energy & Fuels, 2006, 20, pp. 1933-1940. All these related art processes are not suitable for practical industrial applications as they require high temperatures, water, high pressures, pure (99.9%) carbon dioxide streams, and are energy intensive.
In addition to accelerated mineral carbonation studies, several investigators examined aqueous mineral carbonation ex-situ processes to sequester CO2. For example, in one process wollastonite, a calcium silicate mineral, was used to sequester CO2 as described by Huijgen, et al., Mechanisms of aqueous wollastonite carbonation as a possible CO2 sequestration process, Chemical Engineering Science, 2006, 61, pp. 4242-4251. In another related process primary minerals such as calcium, iron, and magnesium silicate minerals were used for mineral carbonation as described by Gerdemann, et al., Ex Situ Aqueous Mineral Carbonation, Environ. Sci. Technol. 2007, 41, pp. 2587-2593. There are drawbacks with these aqueous mineral carbonation processes of silicate minerals as they are energy intensive and require mining, milling, and transport of silicate minerals to a carbonation plant, thereby making them expensive and unsuitable for large scale industrial processing. In addition, these processes require a concentrated source of pure CO2 to work efficiently. In another related art process, coal combustion fly ash and paper mill waste were suggested for use in an aqueous mineral carbonation process as a viable approach to reduce CO2 emissions. See Fernandez Bertos, et al., A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO2, Journal of Hazardous Materials, B, 112, 2004, pp. 193-205; Perez-Lopez, et al., Carbonation of alkaline paper mill waste to reduce CO2 greenhouse gas emissions into atmosphere, Applied Geochemistry, 23, 8, 2008, pp. 2292-2300.
U.S. Pat. No. 5,502,021 describes a related art process that removes contaminants such as purifying exhaust gases, mainly Hg and other nonvolatile metals such as arsenic and selenium, from wastewater. This process requires extensive preparation of a combination of activated reagents. For example, the process requires foaming and slaking with water to increase the surface area of reactive reagents from different sources. This process is also very energy intensive as it requires temperatures between about 150° C. to about 200° C. for its operation.
U.S. Patent Application Publication No. 2005/0002847 A1 describes a related art aqueous mineral carbonation process to sequester CO2 gas. This process uses sorbents in the aqueous system by activating through physical and chemical processes. The physical activation includes reacting sorbents with steam and air at high temperatures of about 300° C. to about 650° C. for about 3 hours, thereby expending a large amount of energy. In addition, the process discloses an activation process by reacting sorbents with a suite of acids and bases for about 4 to about 24 hours. Accordingly, this process is very complex, energy intensive, and requires a pure, concentrated stream of CO2.